1. Field of the Invention
This invention relates to compact lenses, and more particularly to high-range compact zoom lenses suited for photographic cameras or video cameras. Still more particularly, it relates to the reduction of the weight of the entire lens system.
2. Description of the Related Art
The conventional zoom lenses of relatively high range which have found their use in photographic film cameras and video cameras are of the form shown in FIG. 1, where a first lens unit 11 of positive power for focusing, a second lens unit 12 of negative power axially movable in one direction mainly to vary the image magnification from one limit of a range to the other, a third lens unit 13 of negative power axially movable in reciprocating paths convex toward the front to compensate for the image shift resulting from the change of the image magnification, a fixed fourth lens unit 14 of positive power for making afocal the light beam leaving the third lens unit 13, which is not always necessary to use, and a fixed fifth lens unit 15 of positive power having an image forming function are arranged in this order from the front. A diaphragm 16 is, in most cases, positioned in between the third and fourth lens units 13 and 14, or between the fourth and fifth lens units 14 and 15. In the zoom lens of FIG. 1, the second and third lens units 12 and 13 have a common space for zooming movement. Its physical length is, therefore, by nature relatively short. To achieve a further reduction of the size of the entire lens system in the longitudinal and lateral directions, the refractive power of the second lens unit 12 has to be as strong as possible in respect of a reduction of the total zooming movement of the second lens unit 12. The increase in power of the second lens unit 12, however, tends to increase the difficulty of effecting the aberration correction against its zooming movement. There is another problem that the tolerances of the design parameters must be made more rigorous. Besides these, according to the long investigation by the present inventor, it has been found that, in some cases, too much increase in refractive power of the second lens unit 12 calls for an unduly large increase of the axial thickness of the convex lens element to make sure the minimum acceptable edge thickness of the convex lens element is retained. As a result, the physical length of the entire system is contradictorily caused to elongate. Therefore, it is impossible to rely on the method of strengthening the power of the second lens unit 12 to any great extent.
Also, to increase the angular field in the wide angle setting by decreasing the shortest focal length, the diameter of the first lens unit 11 must be increased, because it is determined by the oblique pencil to be transmitted to the extra-axial image point. Hence, the wider the maximum angular field, the larger the diameter of the first lens unit, making the entire lens system more bulky. If the shortest focal length is shifted to the longer side, on the other hand, then because the diameter of the first lens unit 11 is determined by which of, the oblique pencil near or at the wide angle setting and the axial pencil at the telephoto setting has a larger maximum diameter, the longest focal length becomes too long to obtain the prescribed value of the zoom ratio. This is reflected to an increase of the diameter. Another feature of the zoom lens of FIG. 1 is that the second and third lens units 12 and 13 both are negative in power with the axial beam diverging in passing therethrough. Therefore, the aperture diameter of the diaphragm which takes its place behind them tends to increase. The zoom lenses of longer focal lengths generally have larger aperture diameters. Also, the required diameter for the diaphragm unit is usually about 2 times the maximum aperture diameter. Accordingly, it is not desirable to employ the type of FIG. 1 in designing a zoom lens whose longest focal length is too long, when the chief aim is made to achieve an improvement of the compact form of the zoom lens.
It is also known to provide another type of zoom lens shown in FIG. 2 comprising a first lens unit 21 of positive power for focusing, a second lens unit 22 of negative power axially movable in one direction to vary the image magnification from one end of a range to the other, a third lens unit 23 of positive power axially movable in reciprocating paths which, when expanded, follows a locus convex toward the rear to compensate for the image shift resulting from the change of the image magnification, and a fourth lens unit 24 of positive power for forming an image of an object, wherein the diaphragm 25 is positioned in a space between the third and fourth lens units 23 and 24. The use of this type allows for the possibility of making smaller the aperture diameter than that of the former type shown in FIG. 1. But, because the zoom lens units 22 and 23 have no common space for movement, the size of the entire lens system becomes longer in the longitudinal direction. Also, because the distance from the first lens unit 21 to the diaphragm 25 is necessarily increased, the diameter of the first lens unit 21 which is determined by the oblique pencil for the extra-axial image point near or at the wide angle setting tends to increase.
As the related art, mention may be made of a zoom lens having a fixed diaphragm in between the second and third lens units as disclosed in U.S. patent application Ser. No. 877,101 filed June 23, 1986, now U.S. Pat. No. 4,763,998 and assigned to the assignee of the present invention.